The present invention relates to a liquid crystal display, and more particularly relates to achieving a bright display without using a backlight.
Various liquid crystal display elements have been proposed and put to practical use as display elements for displaying information. Currently, a TN mode (Twisted Nematic; Japanese Patent Laid-open Publication No. Sho. 47-11737 (1972) and an STN mode (Super Twisted Nematic; Japanese Patent Laid-open Publication No. Sho. 60-107020 (1985)) are typical modes that are used in nematic liquid crystals, and these modes are widely employed.
In the TN mode, the direction of alignment of the liquid crystal molecules is twisted by 90 degrees when no voltage io applied. Light incident to this element is then polarized by the twisted structure of the liquid crystal and by birefringence, and polarized light is emitted. On the other hand, when a voltage is applied to the liquid crystal layer, the liquid crystal molecules become aligned in the direction of the voltage, less double diffraction occurs and incident light is emitted without being polarized. By adopting a structure where this liquid crystal layer is sandwiched between two light polarizing plates, changes in the optical properties of the liquid crystal layer (changes in the polarized state) can be observed as changes in the strength of the emitted light.
In the STN mode, the direction of orientation of the liquid crystal molecules is twisted by about 240 degrees, which is large compared with the TN mode. Changes in the optical properties of the liquid crystal layer (changes in the polarized state) can be observed as changes in the strength of the emitted light in this mode also by adopting a structure where this liquid crystal layer is sandwiched between two light polarizing plates.
Contrast between light and dark can then be obtained based on this operation theory in the TN and STN modes.
These display methods have the advantages of using remarkably little power, as compared with CRTs (Cathode Ray Tubes), and can be provided in the form of thin display panels. This display method is widely used in information processing equipment, such as personal computers and word processors.
However, because this method employs light polarizing plates as a matter of necessity, about half of the incident light is not transmitted by the liquid crystal display element. In reality therefore, a large number of liquid crystal displays are provided with light sources (backlights) at the rear of the liquid crystal display element to maintain brightness. The amount of light transmitted by the liquid crystal displays provided with color filters for displaying information in color is even lower, and these displays therefore have to be provided with extremely powerful backlights. As the power consumed by the backlight is even greater than the power consumed by the driver circuit for driving the liquid crystal display element, this kind of display is not suitable as a display for portable information equipment that use batteries to provide power. There is then a trade-off in the related liquid crystal display method between brightness and power consumption, and a bright reflective liquid crystal display element that does not require a backlight is therefore preferred.
It is also preferable to obtain a reflective liquid crystal display element without a fluorescent backlight from the point of view of eye fatigue in the case of continued viewing of the display.
Liquid crystal display elements that do not use light polarizing plates have therefore been proposed in response to these problems, of which the "White-Taylor liquid crystal display" is typical (refer to J. Appl. Phys. 45, pp. 4718-4723 (1974)). Here, a cholesteric liquid crystal to which dichroic dye is added is orientated substantially parallel to the faces of the plates. The dichroic dye then efficiently absorbs light because of spontaneous twisting of the cholesteric liquid crystal when there is no voltage applied. Absorption of transmitted light by the dichroic dye does not occur, however, when a voltage is applied because the liquid crystal molecules become oriented in the direction of the voltage (vertically with respect to the plate). In this way, a liquid crystal display element that is both bright and has a high contrast ratio can theoretically be realized without using light polarizing plates. However, it is necessary to make the pitch of the twist of the cholesteric liquid crystal be in the order of the wavelength of light in order to achieve a high contrast ratio, but if the pitch is made short, a large number of line defects, referred to as disclination lines, occur and hysteresis or delayed response is caused. This kind of problem means that widespread use of such a liquid crystal display element is not possible. A PCGH (Phase Change Guest Host) mode has also been proposed as a method of improving the White-Taylor type display element (SAID 92 Digest, pp. 437-440 (1992)). Here, disclination does not occur because the cholesteric twist pitch has been made long. Further, reflectivity when a voltage is applied is made high by carrying out a vertical orientation process at a plate surface boundary. However, the hysteresis is substantial and the absorption efficiency is low when there is no voltage applied because the twist pitch is long compared with the wavelength.
A further typical display method that does not employ light polarizing plates is the PDLC (Polymer Dispersed Liquid Crystal; refer to Japanese Patent Laid-open Publication No. 58-601631 (1983)).
In this method, droplets of nematic liquid crystal of positive dielectric anisotropy that are only a few microns in diameter are dispersed within a polymer matrix. Here, the usual refractive index and the polymer refractive index of the liquid crystal are selected to be approximately the same. Liquid crystal molecules are then orientated in an irregular manner within the droplets when no voltage is applied and directions of orientation vary between droplets. This means that differences in refractive index occur between the droplets and the polymer, with light being scattered as a result. When a voltage is then applied, the liquid crystal molecules become oriented in the direction of the voltage. Scattering of light incident in the direction of the voltage then does not occur because the usual refractive index and the polymer refractive index of the liquid crystal at this time are approximately the same. Light polarizing plates are therefore not necessary for this kind of operating theory. However, the scattering occurring in this mode is not so substantial and a sufficiently bright display is not obtained.
A method of adding dichroic dye to a PDLC liquid crystal (Guest Host PDLC method) has also been proposed (Japanese Patent Laid-open Publication No. Sho. 59-178429 (1984)). Here, the absorption efficiency is low because there is a large number of liquid crystal molecules that are not oriented so as to be parallel with the plate when there is no voltage applied as compared to the White-Taylor method described previously. However, the hysteresis is small compared with the White-Taylor method and the PCGH method and the response is also fast.
Japanese Patent Laid-open Publication No. Hei. 4-199024 (1992) describes a method for improving the absorption efficiency of this Guest Host PDLC method. This technology is configured in such a manner that two guest host PDLC layers with liquid crystal molecules orientated in one direction almost parallel to the plates are overlaid so that the direction of orientation of one layer is orthogonal to the direction of orientation of the other layer. The absorption efficiency is therefore good because the axes for absorbing light between the layers are orthogonal. PDLCs where liquid crystal molecules are oriented in one direction are usually made by extending an ordinary PDLC film in a uniaxial direction. Liquid crystal molecules then also become orientated in this direction because the droplets are also uniaxially extended. However, liquid crystal molecules that are not exactly parallel with the direction of extension also exist within the extended droplets and the absorption efficiency is therefore not good with the White-Taylor method etc.